B2.2: Organelles And Compartmentalization

Question 1

What are organelles?

Answer 1

Discrete structures within a cell that are adapted to preform a specific function

Can be solid structures or combination of dissolved solutes
(Plasma membrane is considered a organelle)

Question 2

What does no/single/double membrane mean?
What are some examples of each?

Answer 2

No membrane: not enclosed by a phospholipid layer-> solid structure in cyto/nucleoplasm
- ribosomes
- centrioles
- nucleolus

Single membrane: enclosed by a single phospholipid bilayer
- Vesicles+vacuoles
- RER/SER
- golgi
- lysosome

Double membrane: enclosed by 2 phospholipid bilayers
- nucleus
- mitochondria
- chloroplast

Question 3

What structures in a cell do not meet the requirements of an organelle?

Answer 3

Cell wall:
Outside of cell membrane (extracellular)
Not in cell -> not organelle

Cytoplasm:
Cytosol -> liquid part of cytoplasm, 80% water and dissolved solutes
Many metabolic reactions happen in cytoplasm -> but is not specialized to preform a specific function -> not organelle

Cytoskeleton:
Composed of many filaments and tubules
Not discrete -> not organelle

Question 4

Outline why post-transcriptional modification of RNA is not possible in prokaryotic cells

Answer 4

In eukaryotic:
mRNA must undergo modification before being translated ->
Parts are removed/altered before leaving the nucleus

In prokaryotic:
No nucleus -> no altering/modifications

Question 5

Explain how eukaryotic cells evolved to have compartments

Answer 5

Through infolding and endosymbiosis -> become super efficient spaces where many activites can happen at once

Membrane is only permeable to small number of substances, barrier between aqueous solutions, a compartment with controlled conditions

Question 6

What are the advantages of compartmentalization?

Answer 6

compartments are separate from environment of surrounding cytosol
Tailored to function of organelle

Enzyme and substrate can be localized -> higher concentration

Damaging substances separated

Optimal conditions maintained for certain processes (efficiency)

Large areas of membrane becomes dense with proteins for a specific process -> location and number of organelles can be altered depending on the requirements of the cell

Question 7

What are the advantages of compartmentalization in the nucleus?

Answer 7

Allow for genes to be separated
- genes that need to be expressed in differentiated cells can be copied and released into the cytoplasm
- genes that don’t need to be expressed -> stay in nucleus/inactive

Question 8

What are the advantages of compartmentalization in the lysosome?

Answer 8

High concentration of enzymes -> can digest/hydrolyse all types of biological polymers

All lysosomal enzymes -> acid hydrolyses -> active at low pH -> maintained in lysosome

Double protection:
compartment allows for protection of other organelles
Neutral pH of cytoplasm would prevent hydrolase from digesting everything

Question 9

Explain phagocytosis/phagocytic vacuole

Answer 9

Phagocytosis:
Process by which a cell uses its plasma membrane to engulf a large particle giving rise to a phagocytic vacuole/phagosome
Type of endocytosis

Cell that does this -> phagocytes
Fuses with lysosome to breakdown pathogen

Question 10

What are the advantages of the double membrane of the nucleus? (Envelope and pore) (what is pore)

Answer 10

(Look on notes (B2.2.6) for info on the nucleus)

The nuclear envelope:
Nucleus surrounded by 2 concentric membranes (inner (INM) and outer (ONM))
Both are phospholipid bilayers
ONM continuous with RER
Protect genetic material in the cell and separates it from substances/chemical reaction in the cytoplasm

Nuclear pores (complex):
Selective passageway where small polar molecules/ions/macromolecules can travel (x30 size of ribosome)
large pore as a result of double membrane
Very big because:
Protein needed for genome structure and function synthesized at ribosomes in cytoplasm -> nucleus
mRNA and tRNA formed at nucleus (transcription) -> exported to cytoplasm
(for translation)
Ribosomes in nucleolus -> into cytoplasm/ER

Question 11

What are the advantages of the double membrane of the nucleus? (Cell division)

Answer 11

(Look on notes (B2.2.6) for info on the nucleus)

Nucleus disassembles and reformes each time (most) cells divide

Prophase -> chromosomes condense and nucleolus disappears -> nuclear membrane fragmented into vesicles -> nuclear pore complex disappears

Final steps of cell division -> vesicles formed by nuclear membrane surround the chromosomes -> vesicles around each chromosome fuse -> double membrane around chromosomes -> vesicles fuses with each other -> complete nucleus and pore complexes reassemble

Question 12

What is the endomembrane system?

Answer 12

System of compartmentalized sacs within a eukaryotic cell that work together to modify processes and ship molecules within/out of the cell

All organelles composed of phospholipids
- phospholipids can move within bilayer:
-> membrane flexible
Which allows for:
Formation of vesicles
Materials to be taken in (endocytosis) or out (exocytosis)
Pinching in of the membrane during animal cell cytokinesis

Question 13

Rough endoplasmic reticulum and ribosome structure (as a part of the endomembrane system)

Answer 13

RER:
Series of connected and flattened membranous sacs with bound ribosomes
Ribosomes synthesize polypeptides and release to the inside of RER
RER -> elsewhere in cell by vesicle -> usually Golgi apparatus

Ribosomes structure:
Made of dozens of proteins arranged on a scaffold of ribosomal RNA (rRNA)
rRNA facilitates the binding of mRNA and tRNA + catalyzes the formation of peptide bonds between amino acids
2 subunits
Once’s assembled -> bind to mRNA and synthesize protein

Question 14

Ribosome structure (as a part of the endomembrane system)

Answer 14

Ribosome -> catalyze and synthesis of polypeptides during translation

Free: floating in cytoplasm and synthesizes polypeptides used in the cell (ex: lysosome)

Bound: attached to RER and synthesizes polypeptides that are secreted from cell or used in the cell membrane

Signal sequence -> dictate if free go to RER or not
Signal sequences occur during beginning of polypeptide -> signal recognition protein bind to polypeptide, pauses translation -> free ribosome bind to a receptor on ER, forms RER -> translation starts again, polypeptide chain moves inside the ER

Question 15

Structure and function of Golgi apparatus

Answer 15

Structure:
Flattened membrane-enclosed sacs called cisternae

Polypeptides synthesized by bound ribosomes on RER -> vesicle -> Golgi
Vesicle fuse with Golgi at cis face (entry face towards the RER/nucleus, convex) -> receives protein/lipid filled vesicles
Polypeptides -> Golgi -> exit via vesicles budding from concave trans face (exit face, usually towards the plasma membrane)

Function:
Polypeptide modified into functional state
- add carbohydrates to make glycoprotein
- combine with other polypeptides to form quaternary structure of protein

After modification -> functional protein

Trans face of Golgi sorts, concentrates and packs protein into vesicles for transport to lysosomes, membrane, exterior, etc.

Question 16

Formation, movement and fusion of vesicles

Answer 16

Membrane-bound sacs used for transport and storage

Formation:
Formed when the membrane bulges and pinches off (result of membrane flexibility/phospholipids being able to move within the bilayer)
-> Golgi, membrane (endocytosis), RER

Clathrin -> protein that creates a coat to help the phospholipids create a rounded shape while the vesicle is forming (very important!)
- coat formed -> receptor proteins on cell surface bind to target molecules -> target molecules are attached -> cytoskeleton protein help clathrin pit to deepen and seal off -> trap target molecule inside

Movement:
Vesicles move through the cell along cytoskeleton track (motor proteins)

Fusion:
Merging of vesicle with another organelle or with part of a cell membrane
Adds phospholipids to the target structure -> larger

Question 17

Function of vesicles (the different types of vesicles)

Answer 17

Transport vesicles:
Transport proteins and lipids from one place to another within the cell

Secretory vesicles:
Transports protein and lipids from inside the cell to the plasma membrane
- integral protein within the plasma membrane (pumps; channels, adhesion/receptor proteins)
- hormones, neurotransmitters

Endocytic vesicles:
Formed by invagination of plasma membrane around an extra cellular substance during endocytosis
Deliver cargo to other organelles for further sorting/digestion (phagocytosis)

Other types include:
Peroxisomes - contain enzymes that digest fatty acids
Lysosomes - contain digestive enzymes which digest cellular waste/harmful substances

Question 18

Explain cell fractionation

Answer 18

Progress in science follows development of techniques -> study of function of individual organelles possible because inventions like cell fractionation

Cell fractionation -> a process that involves a pure sample (contains only the specific organelle being studied) and breaking up a sample of tissues then centrifuging the mixture at different speeds

3 stages:
Homogenization -> cell sample broken up using
homogenizer (blender adjacent)

Filtration -> homogenate is filtered through a gauze

Ultracentrifugation -> filtrate is placed into a tube and tube placed in centrifuge (separates materials based on density)
- speed altered to separate different components of cell based molecular weight

Until this was discovered -> research limited